An airborne EM survey system generally includes a transmitter for generating a primary electromagnetic field that induces eddy currents in the earth. These eddy currents generate a secondary electromagnetic field or ground response. A receiver of the EM system then measures the response of the ground. The currents induced in the ground are a function of conductivity. By processing and interpreting the received signals, it is possible to estimate the distribution of conductivity in the subsurface.
EM measurements can be made in either frequency domain or time domain. In a frequency domain EM system, the transmitter generates a sinusoidal electromagnetic field at one or more frequencies. The amplitude and phase of the secondary field relative to the primary field are indicative of the subsurface conductivity. In a time domain EM system, transient pulses are applied to the transmitter during an ON-period to generate a primary electromagnetic field that induces a decaying secondary electromagnetic field. The receiver measures the amplitude and decay characteristics of the secondary field.
An airborne EM system's signal-to-noise ratio (SNR) is an important indication of the effective depth of exploration of the EM system and its ability to recognize and measure a potential target. Various systems and methods for improving SNR have been known in the art. For example, increasing the distance between the transmitter and receiver may reduce system noise thereby improving the SNR. In time domain systems, increasing the size of the transmitter loop may help increase SNR. However, these conventional improvements are transmitter dependent and usually result in increased overall system size and complexity.
Canadian Patent Application No. 2,748,278 proposes a passive geological surveying system using audio frequency magnetic (AFMAG) technology. The proposed system has a first aircraft towed receiver coil assembly and a second ground-based or airborne receiver coil assembly, wherein the differences in the audio magnetic field measured at the first and second receivers are used to interpret the location of the underground conductors. While this proposed system does not involve an active transmitter, it requires that the two receivers be sufficiently spaced apart, resulting in overall increased system size and complexity in computing the differences between the measurements at different locations.
Therefore, there remains a need for a system that provides improvements to the SNR and/or target discrimination performance of an EM system, independent of the transmitter, and without significantly increasing the overall size and complexity of the system.